2x Converter Question

I've just got a Tamron 2x and wondered about the effect on light. I understand that it reduces light by two stops, but I assume that the only effect is the camera simply increases the ISO and/or reduces shutter speed to compensate. In which case the dof and IQ characteristics should stay the same, i.e. f1.7 will still give me 1.7 dof, not f4 dof (at the new focal length.) Perhaps more importantly, if f8 is the sweet spot for sharpness, then it stays there and doesn't drop to f4. Could anyone confirm this or otherwise please?

Such a simple question and an answer full of pitfalls, but basically a 2x converter will change a 50mm f2 lens (let's take that for simplicity) and turn it into a 100mm f4.

The close focus ability remains the same, so it also becomes a Macro lens focusing to 1:2 or thereabouts.

I think DOF will therefore be as a 100mm lens and the aperture will be the effective aperture, but to be honest I haven't actually tried that out as I'm not fond of the quality loss with teleconverters.

Your f8 "sweet spot" with the 50mm will be rewritten as it becomes f16 anyway and the converter has an unknown effect on quality, but I guess that f8 on the prime lens will remain your best bet because you will be magnifying the best aperture's quality, which will remain the best option.

Now looking at it the other way round, if you take a picture at f2 then to get the same picture with the TC you will need to move twice as far away. It is now f4, the focal length has changed, but the magnification will stay the same. If it was still f2 then the DOF would be unchanged, but it's not, so is the DOF different? Not sure, you'll have to try it and see!